1. Field of the Invention
The present invention relates to 2D image sensors and, more particularly, to an interline transfer type image sensing method and apparatus used for readout of a CCD array image sensor at fast frame rates useful for high speed object profiling and surface geometry measurements.
2. Description of the Prior Art
A typical solid-state imaging sensor device is composed of a charge-integrating photosites arranged in rows and columns. Each photosite responds to incident radiation by providing an electrical signal corresponding to one pixel of frame information. In the interline transfer type, charge is transferred out at once into a parallel structure of vertical shift registers. First, a charge pattern is accumulated at integration sites during image acquisition period, then the charge pattern is transferred into storage columns, also known as vertical shift registers. The transferred charges are then shifted one line at a time, into a horizontal readout register from which a line-by-line 2D video output signal is taken. This is known in the prior art and commonly allows taking pictures at a frame rate of about 30 frames per second.
Also known in the prior art is the use of such CCD cameras in acquiring profiles of objects for surface geometry inspection and measurements of relatively small objects. This is done by illuminating the surface of the object by an incident high energy light plane usually coming from a laser, and capturing the reflected light on the CCD image sensor array. Then, the acquired profile of the object can be isolated for shape or surface geometry inspection of the object. A limitation of the known technique comes from the fact that when many surface portions have to be inspected in a short period of time (e.g. on a assembly-line) the standard frame rate of the CCDs is no longer sufficient, because the known method allows inspecting at most one profile per frame period per CCD camera. Various improvements to the known technique are disclosed in U.S. Pat. No. 4,162,126 to Nakayama et al., U.S. Pat. No. 5,083,867 to David Burk and U.S. Pat. No. 5,177,556 to Marc Rioux.
There are also other prior U.S. patents that allow improvements concerning the frame rate being read from a CCD sensor. This is done by formatting the area sensor into blocks of photosites so that fewer sectors have to be read (e.g. U.S. Pat. No. 4,322,752 to James Bixby) or by adding adjacent pixel charges to improve at the same time the acquisition frame rate and the dynamic range of the image (e.g. U.S. Pat. No. 5,420,629 to Takashi Watanabe). None of these prior patents allow a standard interline transfer CCD to be used having an improvement of a factor greater than 4 over the standard read rate and, when there is improvement, this is done at the expense of detail or quality of the captured image.
In accordance with the present invention, method and apparatus are provided for reading out information from a solid-state image sensor in a fraction of the time that would be required using a known interline transfer CCD sensor for surface geometry inspection or object profiling. Further, according with the present invention, improvement of a factor of 50 is possible over the standard read rate of a CCD area sensor for such particular applications.
According to a broad aspect of the present invention, the charge transfer signal TR, which controls the transfer of the accumulated electrical charges from CCD""s photosites into the vertical shift register is triggered more than once per frame period (e.g. n times per frame period, where n is a value greater than one, while in the prior art method n was always less or equal to one), and this results in a conventionally unwanted overlapped image composed of more than one profile line. Using a filter tuned to the light source frequency or other optical means such as masking, only the profile line of the object being illuminated is captured onto the area sensor in the present invention. The only limitation of the present method is that consecutive profile lines captured during the same frame period do not overlap. In some particular conditions, even overlapped profiles may be isolated by computer calculation.
According to this invention, there is provided a high-speed imaging system which uses an interline transfer charge-coupled device CCD sensor having n horizontal pixel lines, a TR image transfer signal input and a line or vertical transfer signal input. The CCD sensor is arranged to receive profile line images having an essentially dark background and it includes an image acquisition control circuit which comprises first, a vertical transfer signal generator means for generating a signal for the vertical transfer input having a frequency fL of n times per frame period and second, a TR signal generator means for generating an output signal TR having a frequency fTR, for the TR image transfer signal input, wherein the fTR frequency is greater than 1/n*fL, in order to capture more than one profile image per frame period.
It is accordingly an abject of this invention to provide a method of driving an interline transfer type CCD sensor is disclosed, where the CCD sensor has n horizontal pixel lines, a TR image transfer signal input and a line or vertical transfer signal input. The mentioned method comprises the essential steps of (1) generating an output signal of frequency fL of n times per frame period for the vertical transfer signal input in order to transfer CCD vertical shift register charges, one line at a time, into a horizontal shift register and (2), generating an output signal TR having a frequency fTR, for the TR image transfer signal input, wherein the fTR frequency is greater than 1/n*fL, in order to capture more than one profile image per frame period.